mod_http_oauth2: Fix removal of consumed authorization codes Fixes mod_http_oauth2.lua:34: bad argument #2 to 'difftime' (number expected, got nil) The extra preceding argument to :set stored the client-id#code as a value instead of clearing the key, and then later in the periodic cleanup timer this string would be indexed, producing a nil and a traceback

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.